The problem of fluid loss to highly porous underground formations penetrated by a well has, of course, been long recognized. These highly permeable zones are often called thief zones. In water or steam stimulation operations, for example, a serious problem is often encountered because a very small interval of the total production zone may be taking 80 percent or more of the total injected fluid. When this happens, the benefit of the injection project may be lost or greatly reduced.
An isolated high-permeability zone or fracture can be plugged at the well bore face by a shallow layer of applied cement, though such a permanent, relatively irrevocable technique often is undesirable. More desirably, a communicating high-permeability zone is plugged to some considerable depth in order to prevent flood water from otherwise merely flowing around a narrow shallow plug and back into the high-permeability or swept zone. Indepth plugging of a relatively high-permeability zone converts the zone into a much lower permeability zone. Then, subsequently injecting flood water or other fluid will tend to enter the formerly by-passed, but now relatively more permeable hydrocarbon-bearing zones and thus mobilize increased amounts of hydrocarbons therefrom.
Various methods have been used in the past to achieve indepth gelling, such as gelable systems triggered by a following aqueous acidic solution injection for subsequent pH adjustment. However, injecting an acidic solution following the polymer solution may result in gelation occurring so rapidly that a sufficient indepth plugging is not effectively obtained in the most permeable strata where desired. In another method, water, a polymer and a cross-linking agent capable of gelling the polymer such as a sequestered polyvalent metal cation, are admixed, and, just before injection into an underground formation, an acid is added thereto to effect gelation. But, when the acid is pre-mixed with the gelable composition, the gelation can be too fast, making it necessary to shear the gelled polymer in order to be able to obtain adequate injection, which reduces effectiveness of the gel.
Indepth gelling has also been effected by the controlled gelation of sodium silicate. Also, polymers have previously been gelled in permeable zones by borate ions supplied in various ways.
According to this invention, permeability of a highly permeable zone in a subterranean formation is reduced by introducing into the formation, an aqueous solution of a water soluble polymer selected from the group consisting of polyalkylenimines and polyalkylenepolyamines and mixtures thereof and a nonionic polymer which is hydrolyzable to an anionic polymer which is capable of cross-linking with and gelling said water soluble polymers. In one aspect of the invention the nonionic polymer is hydrolyzed prior to introduction into the subterranean formation. The resulting anionic polymer and water soluble polymer are then separately introduced into the formation.